The use of molten glass for the production of glass fibers is wellknown. Glasses used for this purpose contain a multiplicity of chemical elements. Among these is fluorine which is usually introduced into a glass batch as calcium fluoride (fluoro spar) or sodium fluosilicate (sodium silicofluoride). The presence of fluorine in the glass is important because fluorine acts as a melting aid, facilitates fining, and reduces flooding tendencies of the bushing from which glass fibers are drawn.
In calculating the quantity of the fluorine source required to impart the desired fluorine content to the final glass, it is necessary to allow for fluorine losses during the melting process. These losses are in the range of from about 30 to 80 percent of the fluorine added. The fluorine is evolved as silicon tetrafluoride (SiF.sub.4), hydrogen fluoride (HF) and other fluorides such as boron trifluoride (BF.sub.3) and boron oxyfluoride (BOF), depending upon the glass composition. The amount of the fluorine losses depends upon a variety of factors. These fluorine losses result in potential pollution problems.
The method of this invention is directed towards solving this problem.
U.S. Pat. No. 3,331,731 issued to Baak teaches a method of introducing fluorine into glasses which minimizes fluorine loss. Baak's method involves melting all the glass components except the fluorine source to a homogeneous glass, cooling and pulverizing the glass, mixing the pulverized glass with a powdered form of the fluorine source, and melting the mixture to prepare the final glass. Baak's method requires that in the order of 90 weight percent of the final glass batch components be pulverized and remelted to make the final glass.
The method of this invention is also directed at minimizing these costly pulverizing and remelting requirements.